Sensors and sensing systems are used in a number of applications, including surveillance, border protection, environmental monitoring, meteorology, and the like. As microelectronics have become smaller and less power hungry, it has become possible to deploy very small sensors for extended periods of times. For example, autonomous and mobile sensors can be deployed over a wide area to monitor for chemical, biological, or nuclear hazards. When a hazard is detected, it can be reported.
In many sensing systems, it is desirable to determine the location of the sensor making a report. Conventionally, many systems rely on Global Positioning System (GPS) receivers to determine location. In a GPS receiver, signals are received from a number of orbiting satellites, and processing is performed to determine the location of the GPS receiver. Unfortunately, GPS receivers tend to be relatively power hungry for a variety of reasons. While some GPS receiver designs have been developed which are suitable for battery-powered use, they tend to be have various limitations. For example, some receivers cannot operate for more than a few hours without depleting the batteries. Some receivers can be turned off to conserve battery power, but if left off for an extended period of time (e.g., days or months) it can take a long time to resynchronize to the satellites and obtained a position fix. GPS can also suffer problems in that the accuracy of a position fix can be degraded when only a few satellites are within visible range, obstructions are present (e.g., thick forest canopy, buildings, etc.), or when the system is intentionally degraded (e.g., through jamming or enabling of the selective availability).